This is what led to the people saying GL5 oils will kill brass parts. After 100k or so as the fluid was breaking down and as acid content was getting higher GL5 fluids would start to eat away at the soft metal parts to an extent that would cause failures. However, if the oil is changed on a decent interval there are very few problems with GL5 and brass parts.

But according to Redline oil.. GL5 class oils do tend to corrode brass parts because of their properties which lend themselves to protecting differentials.

GL4 oils do not have these additives, and therefore are safer on brass components.

Im not saying the Redline fluid was the cause, Im just saying its def not going to be a fix.

Ya, I'm not expecting it to cure any transmission issues, only to help slow them down compared to a straight GL5 fluid.

Naturally, using a GL4 fluid like MT90 will help gear shifts, but at the cost of less protection for the differentials.

The fact that it was a St165 tranny probably didnt help matters. The 2nd gear syncro on my old E150F was going with only 50k km.

Now here is a question - Does our tranny really need a GL5? Redline says their documents show a GL4 or 5 is suitable for our cars.

Was a GL4 acceptable at one point? then toyota released a service bulletin changing it to GL5 only?

GL5 does have more pressure additives. This is what eats away at the soft metals. However, most GL5s now have additional additives that prevent the pressure additives from eating soft metals.

GL-4; Specified for hypoid gear service under severe service but without shock loading. This classification is essentially obsolete but is still specified by some manual transmission/transaxle manufacturers. Implies an EP/AW additive package that contains 30% to 50% less S-P additives than the GL-5 service classification. Some Marine Gear Lubes fall into this classification, especially the full Synthetic Marine Gear lubes and specialty blenders MT lubes that use high levels of esters.

GL-5; Specified for hypoid gear service but with shock loads and severe service operation. Usually meets Mil-L-2105D and in most cases, is the multipurpose automotive gear oil. Most 75W90 to 75W140 grades meet the GL-5 classification. This grade has a high level of Extreme-Pressure additives that could be mildly corrosive to nonferrous parts, such as brass, bronze and aluminum parts. Most of the modern GL-5 lubes contain metal deactivators that prevents attacks by the extreme-pressure additives. In addition to EP additives, these lubes contain rust inhibitors, defoamants, friction modifiers, thickeners, and Viscosity Index Improvers.

However, I don't know which brands do and don't contain those additional additives, so I play it safe and change my oil.

And I guess I should clarify, GL5 pressure additives would start to eat soft metals after they reached a temp of 250F+ degrees . This is why it usually took over 100k miles to cause any problems. VERY rarely will a street driven car see over 250F but if you add up a little bit at a time it eventually becomes enough to start damageing metals.

Re: Gearbox and diff oil

Fri Jan 15, 2010 4:46 am

hey guys i got some royal purple gear max 75w90 for my torsion rear diff is this the correct weight?i know the manual suggest 80w 90 but give the lowest temps here are 10'c i didnt think the lower weight would matter much.cheers just wanna know before i open the bottle as 2quarts cost me $85aus

Re: Gearbox and diff oil

Re: Gearbox and diff oil

Mon Apr 12, 2010 10:43 am

ok now im filling my gear box and ive read all six pages and it seems every one agrees redline is a great oil but maybe nt the best is this correct?i cant really afford to filll with rp as its around $25a litre here?nt worried bout quick shiffting just want the gear box and syncros to last

Re: Gearbox and diff oil

Tue Apr 13, 2010 6:13 pm

Corey Darling wrote:Hey Mike,

What made you decide to put RP in the diff over redline?

cost and desire to try different things. more cost, because i was buying some other things at a local parts store and it was on sale and they don't carry Redline, only RP, so i said what the hey let me give this a try...